Asphalt-based roofing shingles are commonly made by taking a continuous base sheet which may be of organic felt, or fiberglass mat, covering both sides of the base sheet with a coating asphalt, and then embedding granules on the top side of the coated sheet. The granules protect the asphalt from breaking down through oxidation caused by ultraviolet rays. The finished sheet is cut into lanes and then into desired lengths for shingles.
Since conventional oxidized asphalt which is normally used for roofing shingles has limited strength and life, and can be brittle, it has become common to use a form of asphalt which is referred to as “modified asphalt” for roofing shingles. Modified asphalt has an elastomeric polymer blended with ordinary asphalt. The elastomeric polymer imparts elasticity and greater strength to the asphalt and also improves its resistance to ultraviolet rays.
However, a disadvantage of modified asphalt is that it is extremely expensive as compared with ordinary oxidized asphalt. Therefore, for economic reasons the use of modified asphalt in roofing shingles has been relatively limited. In addition, roofing shingles which use modified asphalt may often be too flexible and can therefore be difficult to apply to a roof. Further, because of their lack of stiffness they may tend to bend upwardly in a strong wind, increasing the likelihood (despite their greater strength) that they may eventually break.